The production and evaluation or monohaploid potatoes (2n=x=12) for breeding research on cell and plant level.

The use of monohaploid potato clones in research in theory has many potential advantages in comparison with the use of heterozygous di- and tetraploids. Because in monohaploids each chromosome is single, recessive as well as dominant alleles are detectable and can be selected for. In addition, monohaploids offer a unique opportunity to produce homozygous clones. The aim of the study presented in this thesis was to determine which are the pros and cons of using monohaploids in practical breeding and fundamental research, and which are the consequences of monoploidy and homozygosity in a normally highly heterozygous crop such as potato.Large-scale production of monohaploids appeared to be possible by prickle pollination, but turned out to be genotype-specific. It was shown that the ability for monohaploid production can be transferred to the progeny by crossing. During the investigation reported herein more than 500 monohaploids have been produced that could be used for further research.In order to combine different selected monohaploid genotypes that will warrant high-level heterozygosity, methods for protoplast isolation, culture and fusion have been adjusted to the material used. Several monohaploid genotypes could be regenerated from protoplasts to plants but always the ploidy level raised to 2x or even 4x. After protoplast fusion, hybrid fusion products could readily be selected because of a differential regeneration capacity. By using isozyme markers for detection of hybridity the first regenerants after protoplast fusion were shown to be hybrids. A difference of at least four weeks in regeneration time was found between the first heterozygous (hybrids) and the first homozygous (parental) genotypes. Some regenerants were triploid. This indicates that at least one monohaploid protoplast was involved in fusion. The majority however was tetraploid and this was probably the result of somatically doubling x+x fusion products. Only in some genotype combinations also increased callus growth rates were found.The plant vigour in homo- and heterozygotes suggests that dominance effects are stronger than additive gene effects. Owing to sterility problems and a relatively bad plant performance, homozygous and there fore also indirectly monohaploid potato clones are of little importance for practical breeding. However, for fundamental research, monohaploids are very useful: (i) they can be produced on a large scale from specific diploid lines, (ii) they can be maintained in vitro on the monohaploid level for at least 2-3 years via shoot tip propagation, and (iii) protoplast isolation and fusion, and plant regeneration from protoplast derived calli has proven to be possible